CVE-2025-12559: 
vulnerability analysis and mitigation

CVE-2025-12559 is an information disclosure vulnerability in Mattermost Server where the platform fails to sanitize team email addresses, allowing any authenticated user to view email addresses that should be restricted to Team Admins only. It affects Mattermost Server versions 11.0.x ≤ 11.0.2, 10.12.x ≤ 10.12.1, 10.11.x ≤ 10.11.4, and 10.5.x ≤ 10.5.12. The vulnerability was published on November 27, 2025, and patches were made available the same day. It carries a CVSS v3.1 base score of 4.3 (Medium) (GitHub Advisory, Mattermost Security).

The root cause is classified as CWE-200 (Exposure of Sensitive Information to an Unauthorized Actor) — specifically, the API endpoint GET /api/v4/channels/{channel_id}/common_teams does not properly sanitize or restrict team email address fields in its response payload. Any authenticated user (low-privilege) can send a standard HTTP GET request to this endpoint and receive team email addresses that should only be visible to Team Admins. No special tools or elevated privileges are required beyond a valid session token, making exploitation trivial for any logged-in user (GitHub Advisory, Mattermost Security).

Successful exploitation results in unauthorized disclosure of team email addresses across the Mattermost instance, impacting confidentiality with no effect on integrity or availability. Exposed email addresses could be leveraged by attackers for phishing campaigns, social engineering, or targeted attacks against team members and administrators. The scope is limited to the affected Mattermost server instance, but the exposure applies to all teams accessible via the vulnerable endpoint (GitHub Advisory, Feedly).

There is no public proof-of-concept exploit code and no evidence of in-the-wild exploitation at this time. The EPSS score is approximately 0.027–0.042%, placing it in the 13th percentile for exploitation likelihood within 30 days. No threat actor attribution or CISA KEV catalog listing has been identified for this vulnerability (GitHub Advisory, Feedly).

1. Authentication: Obtain valid credentials for any user account on the target Mattermost instance (low-privilege account is sufficient).
2. Obtain a session token: Authenticate via the Mattermost API or web interface to retrieve a valid session token or bearer token.
3. Identify a channel ID: Enumerate accessible channel IDs using the Mattermost API (e.g., GET /api/v4/channels or by observing channel URLs in the web client).
4. Query the vulnerable endpoint: Send an authenticated HTTP GET request to GET /api/v4/channels/{channel_id}/common_teams, substituting a valid channel_id.
5. Extract team email addresses: Parse the API response to retrieve team email addresses that are included without proper sanitization, which should have been restricted to Team Admins only (GitHub Advisory).

• Network: Unusual or repeated authenticated GET requests to /api/v4/channels/*/common_teams from non-admin user accounts, especially across multiple channel IDs in rapid succession.
• Logs: Mattermost API access logs showing low-privilege users querying the common_teams endpoint at high frequency or across many channels; look for patterns inconsistent with normal user behavior.
• Behavioral: A single user account querying the common_teams endpoint for channels they are not a member of, or systematic enumeration of channel IDs via the API.

Mattermost has released patched versions addressing this vulnerability: 11.0.3, 10.12.2, 10.11.5, and 10.5.13. Organizations should upgrade to one of these versions immediately. As a temporary measure prior to patching, administrators can restrict API access at the network level and audit user permissions to limit exposure. No configuration-only workaround fully mitigates the vulnerability without upgrading (GitHub Advisory, Mattermost Security).

The vulnerability received limited public attention given its moderate severity rating. It was noted by security tracking services including Tenable (Nessus plugin 278109) and aggregated by Red Hat's CVE database. No significant vendor statements beyond the Mattermost security update page or notable researcher commentary have been identified (Feedly).